From the Recently Viewed Listings Box (Admin Interface -> Settings -> Site Settings -> Recently Viewed Listings Box) menu:
Enable Recently Viewed Listings Box
If checked, enables the recently viewed listings box, which will be displayed (by default) in the footer of the website.
and set (required) an
Enter the number of hours after which a listing will be removed from the recently viewed table.
Users (registered and unregistered) will see the last six item they browsed:
Note: Unregistered users can see their previously browsed items because user preferences are saved in a cookie and stored for 30 days.